With the capability of modern electronic processors to process large masses of information, large capacity storage for data and instructions must be provided. In other words, the need is clearly established for external storage, the capacity of which can be increased at will and in which searching and reading are rapid. These requirements are met by magnetic disc storage units, which typically in large mass storage situations include a cylindrical package of vertically stacked discs.
In magnetic discs, the information is recorded in serial digital form on each face of a disc along a plurality of concentric circular tracks laid down in the circular band between the outer edge of the disc and a central circular area. The tracks may be subdivided into sectors, so that it is possible to select a single segment of track as a storage area for recording or reading. To accomplish this reading and recording, transducer elements are positioned over vertically aligned tracks on the stack of discs.
Rapid reading of the information sought is limited by the availability of means for rapid, accurate positioning of the magnetic transducers relative to the vertically aligned tracks on the stack of discs that contain the information.
Normally, read/write transducers are mounted facing one another on a pair of arms movable in a radial direction with respect to the disc and disposed in the form of a fork so as to operation both sides of the disc. In high capacity disc drives, a plurality of such pairs of recording heads are provided, operating on and facing opposite sides of a stack of discs. Each transducer must be accurately positioned, both in a normal direction with respect to the surface of the disc and in a radial direction with respect to the axis of the disc.
The positioning of the transducer is generally carried out by electromechanical systems. It is obvious that the closer the tracks are brought to one another and the smaller the tolerance in the radial distance between two tracks (with a consequent greater amount of information recordable on the surface of the disc), the greater the problems that arise with the positioning mechanism. The mechanism must not only achieve a low time of access to the information sought, but must also effect precise positioning of the transducer over the desired track. FIG. 1 schematically depicts a group of discs and a corresponding group of data heads such as are typically used in a high capacity disc drive. If the gaps of the recording heads are arranged on a line designated as H.sub.B -H.sub.T (see FIG. 3), they would record information or data on the discs which would ideally form another line D.sub.B -D.sub.T. The two lines should be coincident with each other.
To position the transducers over the desired tracks, the disc drive unit receives from the computer an indication of the track on which the information being sought is located. The disc drive unit must be able to execute the order of the computer by positioning the transducers over the track addressed, and thereafter be able to perform the function of reading or writing (or possibly erasure) which the processor transmits to it.
However, due to dynamic and thermal disturbances which occur in disc drives, the recorded data can become displaced from the data heads. Several typical examples are diagrammed in FIG. 4. The error increases the higher the stack of discs, and the closer the tracks are spaced radially on the disc. The resulting displacement of the recorded data and the head which is trying to read the data represents an error. The maximum error is known as .DELTA.E in the subject figure.
Disc drives often use a servo head to measure the error of the type exemplified in FIG. 4 and then correcting for it by moving the heads a corresponding amount. In known systems, a single servo head is used located at the middle or at one extreme of the disc stack. This solution works well if the error is uniform from one end to the other of the disc stack as shown in FIG. 4, section A. When a tilting, or non-parallel condition exists, as shown in FIG. 4, sections B and C, errors still occur, even after the servo head applies its correction. For example, assume the servo head was at the bottom of the disc stack and the condition shown in FIG. 4, section B occurred. No error would be detected by the servo head and no correction would be applied. However, a significant error could exist at the top of the disc stack, resulting in reading or writing data from a location that differed significantly from that actually addressed.